When mosquitoes bite you, they inject their saliva into your…

When mosquitoes bite you, they inject their saliva into your skin because it acts as an anticoagulant and keeps your blood flowing while they drink. Your body sees the saliva as an intruder and releases histamines that dilate your blood vessels, which allows more white blood cells to attack. This inflammatory response causes swelling – and that’s why mosquito bites itch. source